1. Field of the Invention
This invention relates to a focus detecting device in which a plurality of images are formed on the basis of light beams passing through different portions of the pupil of an objective lens and the focus state of the objective lens is detected from the relative positional relation of these images.
2. Description of the Prior Art
Focus detecting devices of this type are well known and it has been considered to apply them chiefly to single lens reflex cameras.
FIG. 1 of the accompanying drawings shows an example of the focus detecting device of this type according to the prior art. In FIG. 1, reference numeral 0 designates the surface of an object to be photographed, reference numeral 1 denotes a phototaking lens in a single lens reflex camera, not shown, reference numeral 3 designates a field lens provided near the predetermined imaging plane ( the image pickup plane of the camera)2 of the photo-taking lens 1, reference numerals 4 and 5 denote secondary imaging lenses for forming two secondary images on the basis of light beams passing through different portions 1a and 1b, respectively, of the pupil of the photo-taking lens 1, and reference numerals 6 and 7 designate photoelectric conversion element arrays for detecting the secondary images formed by the secondary imaging lenses 4 and 5. The element arrays 6 and 7 each are comprised, for example, of CCD (charge coupled device) or the like. In this device, when, for example, the photo-taking lens 1 is moved leftwardly as viewed in FIG. 1 to provide a front focus state, the images of the object to be photographed formed on the light-receiving surfaces of the respective photoelectric conversion element arrays 6 and 7 by the secondary imaging lenses 4 and 5 deviate in the directions of arrows and thus, the front focus state and the amount thereof are detected from the variations in the outputs of the photoelectric conversion element arrays 6 and 7 corresponding to the relative deviation of these images. Also, in the case of the rear focus state, the respective images deviate in the directions opposite to the directions in the case of the front focus state and thus, the rear focus state and the amount thereof are detected.
Now, the field lens 3 is for causing the pupils 4a and 5a of the secondary imaging lenses 4 and 5 to be imaged on different pupil portions 1a and 1b of the photo-taking lens 1 and keeping the uniformity of the quantities of incident light on the light-receiving surfaces of the photoelectric conversion element arrays 6 and 7, and is important for improving the detection accuracy of such device, but generally the field lens is required to be of a simple construction for reasons such as the space in the camera, the cost and the convenience of manufacture and therefore, a single lens is often used as the field lens 3. Accordingly, in this lens, aberrations have been relatively great and it has been difficult to make the pupils 4a and 5a of the secondary imaging lenses 4 and 5 completely coincident with the pupils 1a and 1b, respectively, of the photo-taking lens 1.
Therefore, in such a focus detecting device, the so-called shading occurs and even for a surface of uniform brightness, the non-uniformity of the quantity of light as indicated by solid line, for example, in FIG. 2 of the accompanying drawings remains on the light-receiving surfaces of the photoelectric conversion element arrays 6 and 7. FIG. 2 shows the distribution of quantity of light on each element array when the light-receiving surface of each photoelectric conversion element array 6, 7 shown in FIG. 1 is seen from the direction of arrow A. Generally, non-uniform distribution of quantity of light is a distribution in which the left and right have been reversed in the relative relation of the photoelectric conversion element arrays 6 and 7, while on the element arrays, it is a distribution asymmetrical with respect to the center lines 6a and 7a of the light-receiving surfaces of the element arrays and therefore, when a point P provided on the object surface 0 of FIG. 1 is considered, the quantities of light at corresponding points P1 and P2 on the element arrays differ from each other, and this may result in reduced detection accuracy of the amount of deviation of the image.
As a result, the device of this type according to the prior art has suffered from the problem that accurate focus detection is difficult.